The present invention generally relates to systems for filtering out noise from electrical signals produced by mass/weight measurement apparatus. More particularly, the invention relates to reduction of sensitivity of a load cell to environmental vibration. Yet more particularly, the invention relates to systems for filtering out noise from electrical weight signals generated by devices for vacuum assisted transfer of fluids.
The weight signal produced from a mass/weight measurement device such as a strain gauge load cell or force balance generally includes undesirable components attributable to environmental influences. Such influences can include vibration, loading shock, or the like.
Typically, the component of interest in the weight signal is of a sufficiently low frequency that low pass filtration can be employed to remove many of the undesirable components. However, it is difficult to completely remove all undesirable components.
Certain filtration arrangements are disclosed in U.S. Pat. Nos. 4,817,026 and 4,379,495. However, the arrangements are not suitable for all occasions. Specifically, the disclosed filtration arrangements are applied in situations where the signal components of interest are static, i.e., constant or nearly constant, DC values.
In hospitals, it is frequently necessary to provide solutions for intravenous administration to a patient which contain a variety of drugs in a single solution container. A common example of such need arises when a patient is receiving all of his nutritional needs intravenously. In this situation, the patient will typically receive a basic solution containing amino acids, dextrose, and fat emulsions which provide a major portion of the patient's nutritional needs. However, the solution is insufficient to maintain a patient for an extended period of time. Therefore, a typical total parenteral solution contains as many as 8 to 12 additional additives. The additives are typically minute quantities of vitamins, minerals, electrolytes, etc.
An automatic compounding device has been developed to assist the pharmacist in preparing solutions for total parenteral nutrition. This devices is disclosed in U.S. Pat. No. 4,789,014, the teachings of which are fully incorporated herein by reference. The disclosed device is used to assist the pharmacist in automatically compounding very small quantities of fluids into a single receiving container. The device allows the pharmacist to accurately transfer multiple individual fluids for multiple source containers into a single receiving container.
In the device, fluid flows from multiple source containers through individual fluid supply conduits into a measuring chamber having a single fluid outlet conduit in fluid communication with a single receiving container. The measuring chamber comprises a load cell that weighs minute quantities of fluids as they are added to the measuring chamber.
The weight signal generated by the load cell of the compounding device is such that it is either a constant value (i.e., a DC output signal) or a ramp wave (i.e., a very low frequency AC output signal). Thus, the signal components of interest in the weight signal are variable.
Further, due to the minute quantities being measured, the weight signal is particularly susceptible to vibrational influences. The weight signal can be highly susceptible to degradation in the presence of environmental vibration, loading shock, and the like.